Oxidation process



United States Patent 3,546,082 OXIDATION PROCESS John Rickard Mansfield,Norton-on-Tees, England, assignor to Imperial Chemical IndustriesLimited, London, England, a corporation of Great Britain No Drawing.Filed Apr. 29, 1968, Ser. No. 725,181 Claims priority, application GreatBritain, May 5, 1967, 21,027/ 67 Int. Cl. C07f 15/00; B01k N00 US. Cl.204--72 7 Claims ABSTRACT OF THE DISCLOSURE A Group VIII noble metal,e.g., palladium, is converted to a carboxylate by electrolysing a slurryof the metal in the anode compartment of a cell comprising anode andcathode compartments separated by an anion exchange membrane or saltbridge. The cathode compartment contains an electrically conductingcarboxylic acid solution and the noble metal carboxylate is withdrawnfrom the anode compartment.

The present invention relates to an oxidation process, particularly tothe oxidation of a metal from a lower to a higher valency state.

Processes are known in which the noble metals of Group VIII of thePeriodic Table (platinum, palladium, rhodium, iridium, ruthenium andosmium) are used to oxidise olefinic compounds to aldehydes, ketones,unsaturated esters, unsaturated ethers or acetals. Such processes aredescribed in British patent specification No. 964,001 and our co-pendingU.S. applications Ser. Nos. 601,868 filed Dec. 15, 1966 and 646,490filed June 16, 1967. In carrying out the oxidation the noble metal isreduced to the zero-valent form. The vero-valent form of the metalcannot be re-oxidised directly by molecular oxygen but only by theintervention of a redox system which is suitably a metal salt ofvariable valency, for example, a copper, iron or cobalt salt. Thus inoxidising the zero-valent noble metal the redox system is reduced to itslower valent form which may then be re-oxidised with molecular oxygen.In the absence of the redox system the noble metal on reduction isprecipitated in metallic form.

In processes such as those mentioned above, the noble metal used may bepalladium in the form of its carboxylate, for example palladous acetate.The redox systems employed may also be metal carboxylates, for examplecopper acetate, iron acetate or cobalt acetate.

In carrying out such processes however, despite the presence of a redoxsystem and molecular oxygen, a precipitate of metallic palladium maybuild up after several hours of continuous operation. This precipitatemay also contain cuprous salts which have precipitated without beingre-oxidised to the cupric form.

It is known that while the dissolution of palladium in hydrochloric acidis readily effected its dissolution in carboxylic acids such as aceticacid is very difficult to achieve.

The present invention provides a process whereby a Group VIII metal suchas palladium may be reconverted to a corresponding carboxylate. It alsoprovides a process whereby the reduced form of a metal carboxylate redoxsystem may be reconverted to the oxidised form. In one form the processof the present invention may be used in conjunction with the processesdescribed in our specifications referred to above, to re-oxidise thepalladium salt directly without the use of a redox system and molecularoxygen, or to re-oxidise the redox system without the use of molecularoxygen or to facilitate the re- 3,546,082 Patented Dec. 8, 1970oxidation of the palladium salt by molecular oxygen with or without theuse of a redox system.

According to the present invention, an oxidation process comprisesintroducing a slurry of a finely divided noble metal of Group VIIIand/or a slurry or solution of the reduced form of a metal carboxylateredox system to the anode compartment of an electrolytic cell comprisinganode and cathode compartments separated by an anion exchange membraneor a salt bridge, the cathode compartment containing a cathode and anelectrically conducting carboxylic acid solution and the anodecompartment an anode, applying a direct current between the anode andcathode, and withdrawing a solution of a noble metal carboxylate and/orthe oxidised form of the metal carboxylate redox system from the anodecompartment.

The noble metals of Group VIII are palladium, platinum, rhodium,ruthenium, osmium and iridium.

Hydrogen is liberated at the cathode in the process of the invention andif oxygen is supplied at the cathode the hydrogen may be converted towater with consequent liberation of energy which may usefully help inreducing the overall electrical energy required by the process.

The slurry or solution which is introduced to the anode compartment maybe formed in a separate reaction zone. It may alternatively be producedin situ by oxidation of an olefinic compound as described in the abovementioned co-pending applications using the anode compartment as theoxidation reactor. The molecular oxygen which is a feature of the abovedescribed processes would however be used in the cathode compartment toconvert the hydrogen formed to water.

The solution or slurry introduced to the anode compartment may compriseany suitably inert electrically conducting liquid. Suitably the liquidis a carboxylic acid for example acetic acid. As the slurry or solutionmay have been produced in the catalytic oxidation of an olefiniccompound it may also contain other catalyst components such as alkalimetal or alkaline earth metal salts, e.g. lithium acetate, nitrogencompounds such as oximes, Water and organic reactants and products.Other elemental metals may also be present in the slurry e.g. metalliccopper and/or other metals such as metallic cobalt derived from metalcarboxylate redox systems.

The dissolution of the noble metal is also improved by deliberatelyincorporating a complexing agent in the anode compartment solution. Thisagent, by complexing with the noble metal improves the solubility of thenoble metal ion and facilitates electron transfer. Nitriles such asacetonitrile and benzonitrile, amides such as acetamide, metalcarboxylates such as lithium acetate and, most preferably, oximes andnitrates, nitriles, nitrosyl and nitroso compounds such as are describedin our copending US. application Ser. No. 601,868 filed Dec. 15, 1966are suitable. Acetoxime, acetaldehyde oxime and formaldoxime arepreferred complexing agents. The concentration of the complexing agentmay suitably be in the range 0.01 to 1 molar.

The cathode compartment contains an electrically conducting carboxylicacid solution. Preferably the carboxylic acid contains up to four carbonatoms and more preferably is acetic acid. As will be discussed later thecarbon number of the acid which is used dictates to a certain extent thedesign of the cell. To improve the conductivity of the carboxylic acidit is advantageous to include carboxylate ions other than those providedby self-ionisation of the carboxylic acid. The carboxylate ions arepreferably provided in the form of an alkali metal or alkaline earthmetal carboxylate, for example sodium or lithium acetate. Theconductivity may be further improved by the presence of water. Suitablecarboxylic acid solutions for use in the cathode compartment comprise,for example:

Percent by wt. Alkali metal or alkaline earth metal carboxylate 0.1-2.0Acetic acid 65-96 Water 35-4 As carboxylate ions are continuouslyremoved from the cathode compartment it is advisable when operating theprocess continuously to add carboxylic acid to the cathode compartmentduring the process to maintain the carboxylic acid concentration.

An electrochemical cell suitable for use in the above process comprisesanode and cathode compartments separated by an anion exchange membraneor a salt bridge the cathode compartment containing a cathode and theanode compartment an anode, a direct current source being connectedbetween the anode and cathode.

The cathode may be constructed of any suitable conductor of electricitywhich is chemically inert to the electrolyte. Thus cathodes made ofcarbon, copper, nickel or iron may be used. The cathode may be rod-likeor plate-shaped or may be formed as a cylinder surrounding the anode.

The anode is preferably made of carbon and is preferably porous to givethe maximum surface area. In an improved form of the cell the anodecompartment contains a plurality of carbon granules preferably of 0.5 tomm. diameter. The anode compartment is preferably substantially filedwith such granules.

The salt bridge may comprise any concentrated solution of carboxylateions corresponding to the carboxylate it is desired to produce. Thecarboxylate ions may be provided by an alkali metal carboxylateparticularly a lithium carboxylate.

The anion exchange membrane may comprise any suitable exchangerparticularly one containing quaternary nitrogen atoms. The membrane maybe heterogeneous e.g. finely ground anion exchange resin deposited on athermoplastic matrix, or homogeneous, e.g. formed from the condensationproduct of a suitable nitrogen-containing compound and formaldehyderolled out into a thin sheet before setting. It may be advantageous toprovide a support for the membrane to prevent distortion. For examplewhen the cathode is a plate and the cell is of a sandwich construction aperforated polytetrafluorethylene spacer may be placed between thecathode and the membrane. The carbon number of the carboxylic acid useddictates the choice of membrane as carboxylate ions have to pass throughthe membrane from the cathode to the anode compartment. As the molecularsize of the carboxylate ion increases the passage becomes slower andmore difiicult.

The voltage required to be supplied by the direct current source is afunction of the cell construction and may be determined by simpleexperiment. Advantageously the voltage is maintained at the valuerequired to oxidise the metals present and is not allowed to risesubstantially in excess of this value as secondary oxidations of organicmaterial present may take place.

A preferred form of the cell is one in which the anode compartment iselongated and filled with carbon granules to form a packed column. Thegranule size is chosen by reference to the nature of the slurry and 1 to2 mm. diameter granules have proved suitable in many instances. Theanode may be a rod running down the centre of the column and the cathodea cylinder surrounding the anode compartment being separated therefromby a cylindrical anion exchange membrane. Such a design assists therecovery of the oxidised product from the anode compartment as it may becollected in the liquors percolating through the column.

The process of the present invention is particularly applicable to thefollowing oxidations of olefinic compounds utilising noble metal ofGroup VIII catalysts particularly a palladium catalyst.

(1) The oxidation of olefines in the presence of carboxylate ions tounsaturated esters, e.g. ethylene to vinyl acetate and propylene toallyl acetate.

(2) The oxidation of allyl acetate to esters of glycerol as described inBritish patent specification No. 987,278.

(3) The oxidation of olefines in the presence of water to aldehydes orketones e.g. ethylene to acetaldehyde and propylene to acetone.

(4) The oxidation of olefines in the presence of alcohols to ethers andacetals e.g. ethylene to methyl vinyl ether and dimethylacetal.

In the above processes palladium carboxylates, particularly palladousacetate, may be used as catalysts. The redox system may be a copper oriron carboxylate, e.g. copper acetate. In our co-pending US. applicationSer. No. 622,430 filed Mar. 13, 1967, now abandoned, combinations ofredox systems e.g. copper and cobalt carboxylates are disclosed for usein the above processes and such combined redox systems may bere-oxidised by the process of the present invention.

EXAMPLE 1 The cell comprised a carbon rod 4" long and diameter as theanode surrounded by a cylindrical anion exchange membrane forming ananode. compartment 1 /2" in diameter. The ends of the cylinder weresealed, glass tubes being provided to allow the introduction of liquidto the compartment and withdrawal of liquid from the compartment. Thecathode comprised a cylinder of copper foil surrounding the membrane andsupported in a glass tube, the ends of the tube being sealed to form thecathode compartment. The diameter of the cathode compartment was 3". Asource of DC. current comprising a rectifier connected to the A.C. mainssupply was connected between the anode and the cathode. The cathodecompartment was filled with an electrolyte comprising:

Lithium acetate0.8 mole/litre Water20% by volume Acetic acid-% by volumeThe anode compartment was filled with a mixture of 50 mls. BritishStandard 10 to 16 mesh carbon granules and finely powdered metallicpalladium.

milliamps of current at 4 volts was delivered to the cell whileelectrolyte of the above composition was introduced into the anodecompartment and allowed to trickle through the carbon granules.

The solution of the electrolyte withdrawn from the anode compartmentcontained 4 grams/litre of palladous ions as palladous acetate. The rateof dissolution of palladium was 1 mole per cubic meter of carbongranules per hours.

EXAMPLE 2 The cell comprised a rectangular sandwich of crosssectionalarea 6" x 3 consisting of, in order, a A" polytetrafluoethylene backplate with two inlet ports cut in its face, a A" carbon cathodesimilarly equipped with two parts coincident with those in the backplate, a A" polytetrafluoethylene spacer with a cut away centre portionforming a cathode compartment, an anion exchange membrane, a /1."polytetrafluoethylene spacer with a cut away central portion or anodecompartment filled with carbon granules and having inlet and exit portson its shorter side edges, a carbon anode and a polytetrafluoethyleneback plate. The whole was held together by longitudinal bolts. A sourceof DC current comprising a rectifier connected to the A.C. mains supplywas connected between the anode and the cathode.

The cell-sandwich" was laid horizontally and the cathode compartmentfilled with electrolyte comprising:

Lithium acetate-0.1 mole% litre Acetic acid20% by volume Water-80% byvolume A slurry of finely divided metallic palladium in a solution ofthe above composition was also fed into the anode compartment via theupper port. The solution emerging from the lower port was recycled tothis compartment.

The temperature was ambient and 100 milliamps of current at 4 volts wasdelivered to the cell.

The rate of dissolution of the palladium was 3.0 moles per cubic meterof carbon granules per hour.

EXAMPLE 3 Example 2 was repeated incorporating 0.1 molar concentrationof acetoxime in the electrolyte in the anode compartment. The rate ofdissolution of the palladium increased to 7.6 moles per cubic meter ofcarbon granules per hour.

EXAMPLE 4 Example 1 was repeated employing a mixture of metals ofcomposition metallic palladium 1.7%, metallic cobalt 44.4% and metalliccopper 53.9%. The solution obtained contained 0.1% palladous ions, 0.06%cobaltous ions and 0.2% cupric ions.

I claim:

1. An oxidation process which comprises passing a direct electrolyzingcurrent through at least one member of the group consisting of a slurryof a finely divided noble metal of Group VIII, a slurry of the reducedform of a metal carboxylate redox system from the group consisting ofcopper, cobalt and iron and mixtures thereof and a solution of a reducedform of said metal carboxylate redox system in an anodic zone, and anelectrically conducting carboxylic acid solution in a cathodic zone,permitting carboxylate anions but not cations to pass from said cathodiczone to said anodic zone, the current being sufiicient to convert noblemetal in said noble metal slurry to the carboxylate and the reduced formof the redox metal carboxylate to the oxidized form thereof andwithdrawing at least one member of the group consisting of a solution ofa noble metal carboxylate and a solution of the oxidized form of themetal carboxylate redox system from said anodic zone.

2. The process according to claim 1 in which the slurry or solutioncomprises a carboxylic acid and the noble metal is palladium.

3. The process according to claim 2 in which the carboxylic acid in thecathiodic zone contains up to four carbon atoms.

4. The process according to claim 3 in which the solution in thecathodic zone contains carboxylate ions other than those provided byself-ionisation of the carboxylic acid.

5. The process according to claim 4 in which the solution in thecathodic zone contains water.

6. The process according to claim 1 in which the solution in the anodiczone contains a complexing agent.

7. The process according to claim 6 in which the complexing agent is anoxime.

References Cited UNITED STATES PATENTS 3,119,874 1/1964 Paszthory et al.260-597B 3,303,020 2/1967 Clement et al. 260597BX 3,365,498 1/1968Bryant et al. 260597BX JOHN H. MACK, Primary Examiner I. C. EDMUNDSON,Assistant Examiner us. 01. XJR. 260-597

